The present invention relates generally to an integrated services digital network (abbreviated ISDN for short) terminal, an ISDN terminal communication system, and a method for controlling ISDN terminals. The present invention can be effectively applied to a facsimile apparatus used for the ISDN.
With the recent advances in ISDN construction, facsimile apparatuses used for the ISDN have been accordingly developed. The ISDN is supposed to replace the public services telephone network (called PSTN for short), however, until this actually happens, it is necessary to reconcile the use of the ISDN with that of the PSTN, and thus information must be transmitted between a terminal of the ISDN and that of the PSTN. Hereupon, a facsimile apparatus has been developed to serve as a group 4 facsimile apparatus, which is mainly used for the ISDN, and a group 3 facsimile apparatus, which is mainly used for the PSTN. These facsimile apparatuses use multichannel basic interfaces for the ISDN, such as two data channels (called B-channels hereinafter) and a one signal channel (called D-channel hereinafter). The B-channels are mainly used for data transmission, and the D-channel is mainly used for call control. Therefore, a data transmission through one of the B-channels can take place almost simultaneously with the acceptance of a call through the D-channel. And the B-channels are usually controlled by a single transmission controller so that the data transmission through one B-channel cannot take place simultaneously with data transmission from the other B-channel. Additionally, a facsimile apparatus ignores or rejects an incoming call based on an outgoing call from a terminal while it is communicating with another terminal since one of the B-channels is being used. Consequently, the terminal transmits the same outgoing call to the facsimile apparatus after the communication is finished. Therefore, the outgoing call must be unnecessarily repeated. On the other hand, a facsimile apparatus which can control both B-channels simultaneously is expensive.
Hereupon, a receiver generally transmits a receipt result to a transmitter via one of the B-channels after the receiver has received a predetermined data from the transmitter. Therefore, if the transmitter is communicating with another facsimile apparatus, the receiver cannot effectively transmit the receipt result. In particular, a general facsimile apparatus is limited as regards repeated outgoing call times so that the receiver sometimes fails to communicate with the transmitter in a case where the transmitter is communicating for a long period of time.